The long term goal of our proposal is to understand the mechanisms by which transfusion of older pRBCs units worsens patient outcomes after resuscitation from hemorrhage. Hemorrhagic shock is the most common cause of potentially preventable death after trauma. Current resuscitation strategies for patients with significant blood loss include the use packed RBCs (pRBCs) and fresh-frozen plasma. While the use of pRBCs for resuscitation of the injured patient is essential for survival, transfusion of pRBC units that have aged during storage is associated with worsened clinical outcomes in patients, including increased risk of multisystem organ failure, pneumonia, renal failure, sepsis, and death. We have recently demonstrated that aged pRBC units contain red blood cell microparticles. Our previous studies and preliminary data strongly indicate that these microparticles are critical mediators of the inflammatory response and lung inflammation after resuscitation from hemorrhage. In the current proposal, we hypothesize that microparticles from stored pRBCs are pro-inflammatory in nature and promote inflammatory consequences, such as the development of lung inflammation, after resuscitation from hemorrhage. To test this hypothesis, we propose the following specific aims. Aim 1: Determine the nature of the formation and biochemical composition of microparticles during storage of pRBC units. Aim 2: Determine the molecular mechanisms of cellular activation by RBC derived microparticles. Aim 3: Determine the manner in which RBC microparticles promote multi-cellular interactions leading to organ inflammation after hemorrhage and resuscitation. The proposed studies will generate novel data concerning the role of microparticles in the inflammatory sequelae due to the use of stored pRBCs for resuscitation from hemorrhage and will provide important new information to guide the development of therapeutic strategies to mitigate organ injury after hemorrhage and resuscitation.